During flight operation, a turbine engine of an aircraft can be represented with regard to a flow analysis, to a good approximation, as a point sink that sucks in air. In this context, it is possible to achieve a nearly constant flow velocity over the entire surface area of the intake cross-section by appropriately forming or configuring the intake contours. Such a constant axial flow velocity over the entire cross-sectional intake area is necessary in order to provide an optimal air flow to all of the blades of the compressor stages of the engine, and particularly to avoid flow separations. In the event that external influences disturb the flow velocity profile in the engine intake, then an optimal air flow will no longer be provided to the blades of the first compressor stage in the area in which the flow velocity profile has been deformed or disturbed. If, as a consequence, the apparent free stream velocity of the air flow relative to the blades in this area varies beyond an acceptable limit, then at least partial flow separations will arise at these areas, and can lead to complete blockage of individual blade channels and can possibly also lead to unacceptable vibrations of the individual blades.
While essentially only lateral wind gusts and the like will have the above described negative influences on the operation of the turbine engine of an aircraft during flight, additional negative influences will have further effects on a turbine engine that is ground-based, i.e. operating on or near the ground. Such additional influences that can lead to unacceptable flow conditions include the ground effect for any turbine engine operating near the ground, excessive lateral wind effects such as near-ground wind gusts and the like, and the air flow disturbing effects of any stand or supporting arrangement that supports the ground-based turbine engine. Throughout this specification, the term "ground" is intended to mean any non-air-permeable surface that is stationarily fixed to the earth, such as tarmac, turf, soil, concrete, steel decking, wood decking, any testing facility floor and the like. Also, terms like "ground-based" and "near the ground" mean close enough to the ground that a ground effect will have an influence on the air flow into the air intake.
These other influences can take effect in the turbine engines of aircraft when the aircraft are operating on the ground, either stationary or taxiing. Moreover, these other influences can be effective on the turbine engines of ground-based vehicles such as tanks and other military vehicles, boats, and land-based vehicles powered by turbine engines. Most significantly, these other influences are effective on stationary ground-based turbine engines, including permanently installed stationary engines of a power plant or the like, and turbine engines that are temporarily stationarily arranged in a test stand for conducting a static test of the respective engine. In this latter case, the test stand itself may induce oscillations or variations of the air flow field due to non-stationary separations of the air flow along the edges or rims of the test stand itself.
FIG. 1 shows a conventional or prior art arrangement of a turbine engine 5 operating in a stationary arrangement relative to the ground 3. FIG. 1 particularly represents the case of a turbine engine mounted on a test stand for carrying out a static test, but also applies to any situation in which the turbine engine is operating, especially in a stationary manner, near the ground. FIG. 1 especially shows the ground effect that generates a spiral vortex 2 with a vortex core 4, which is formed on the ground 3 a small distance in front of the air intake 1 of the engine 5, and which extends up to and enters into the intake 1 at the bottom portion thereof.
The underlying cause of the generation of such spiral vortices 2 is the superposition of a rotationally symmetrical sink flow with a rotational air flow that disturbs or interferes with that sink flow. In the case of the air flow into the engine intake 1, the symmetry of the sink flow being sucked into the intake 1 is interrupted and disturbed by the ground 3. As a result, the air flow in the vicinity of the ground 3 tends to have a rotational impulse or momentum imparted thereto. If the air particles in the vicinity of the ground 3 have even a slight rotational momentum imparted thereto, this rotational momentum will become ever stronger as the respective air particle approaches closer to the engine intake 1. As a result, the angular velocity of the air particles in the region of the spiral vortex 2 becomes very large, and thus stabilizes and perpetuates the vortex system.
If a highly turbulent wind flow is further superimposed on the above described flow system formed by the superposition of the sink flow and the spiral vortex flow, then very great flow instabilities can be formed in the region of the spiral vortex. This results in an irregular alternating sequence of collapse and reformation of the vortex 2. In other words, while the spiral vortex 2 either can be formed or cannot be formed, without variation, under conditions of a uniform, constant, turbulence-free lateral wind impingement, on the other hand, the turbulence in the natural atmospheric wind flow will result in momentary collapse of the vortex 2 and then re-forming of the vortex 2. Thus, the interaction of the ground effect with the turbulent atmospheric wind flow is the predominant basic cause for the observed instability of the intake air flow of turbine engines during static tests and other ground-based operation. Moreover, this natural instability of the air flow can even be increased or amplified because the structural configuration of the test stand for conducting the static test can cause varying and non-stationary separations of the wind flow along the edges or rims of the test stand and associated equipment.
The above described instabilities of the engine intake air flow of turbine engines mounted on static test stands, especially under the influence of lateral wind gusts and the like, are known in the art and have already been noted to cause critical operating situations of turbine engines being tested in various static test stand arrangements. These critical situations have even led to so-called engine stalls. After the occurrence of an engine stall, i.e. an air flow condition resulting in an aerodynamic stall of rotor blades of the engine, it is necessary to dismount and disassemble the engine and carry out an extensive, very expensive inspection for damage.
In order to avoid such instabilities and the resulting unacceptable operating conditions, it has been long and widely attempted in the art to smooth out and uniformalize the wind flow into the intake or inlet opening to the test stand arrangement. A known measure in this context is, for example, the arrangement of deflector vanes in the inlet or intake area of the test stand arrangement. However, it is also known in the art that such measures have previously been unable to achieve a satisfactory solution of the above described problems.